Displacement pumps may require substantial force for control of a movable mechanical element which determines the displacement of the displacement pump--typically a swash plate or an eccentric ring. To permit remote control of the displacement of such pumps, it has previously been proposed to provide electrical control units which, selectively and through valves, admit pressure fluid to pistons acting on the movable mechanical element. It has been proposed to mechanically connect a manual lever with the movable mechanical element which, additionally, controls the slider of a control valve, for example of a spool valve. The manual lever is positioned in accordance with a desired deflection of the mechanical movable element and, in accordance therewith, a pressurized hydraulic fluid is controlled by the control valve to move the moving piston connected to the mechanical element to deflect the element in the desired direction. The mechanical connection of the element with the manual lever on the one hand, and with the slider of the control valve on the other hand forms a closed loop so that the slider of the control valve will interrupt admission of additional pressurized hydraulic fluid to the positioning piston after the mechanical element, typically the swash plate or the eccentric ring, has reached the desired position. It is thus possible to change the position of the mechanical element by a predetermined adjustment of the manual lever based on a mechanical follower system by any desired amount.
It is frequently desired to provide for remote control of such displacement pumps. It has thus been proposed to control the admission of pressurized fluid to the adjustment pistons of the pumps by means of an electronic control unit. The inclination or eccentricity of the respective mechanical element is sensed by a position transducer and the actual position is then applied to a control unit, together with a commanded position. An error signal is developed which then is applied as a control signal to hydraulic valves to admit pressurized hydraulic fluid to the adjustment pistons or, respectively, drain pressurized fluid therefrom.
The electrical control is particularly applicable for remote control installations. It was not possible heretofore, however, to combine the reliability of the hydro-mechanical adjustment and the flexibility of electronic control, which additionally permits remote control and, therefore, if design requirements made remote control necessary, it was not possible to provide a system which could continue to operate even if defects in the electronic control system would arise.